I have a theory about how Roronoa Zoro got his name as “Zoro”. First, I think Zoro belongs to the fabled Shimotsuki Clan, as this was clear from his home village name and his birth month November (which is called Shimotsuki in Japanese).
It’s highly likely that Zoro’s father was Shimotsuki Ushimaru, the daimyo of Ringo. He ruled until the takeover of Kurozumi Orochi as shogun 20 years ago. He was a direct descendant of the legendary samurai Shimotsuki Ryuma.
Here is my theory, Shimotsuki Ushimaru planned to name his unborn son as Onimaru after his trusted partner and pet Onimaru, the fox. His wife is also aware of his wish to name their son Onimaru.
Twenty years ago, Kurozumi Orochi assumed the title of shogun, and after executing the daimyo of Kuri Kozuki Oden, he gave the remaining four daimyo the chance to submit to him. The daimyo all refused and staged a rebellion against him. The rebellion was ultimately quashed by Kaidou, as Ushimaru and his fellow samurai ended up imprisoned on Onigashima not long after the end of their rebellion. Around that same time, Kaidou locked his daughter Yamato in a cave along with the samurai prisoners for a month or until they agreed to finally obey him.
Ten days later the samurai decided to break out and fight Kaidou immediately to ensure that Yamato did not die in the cave as Kaidou wanted and that she survived until the day of Oden’s prophecy. This ultimately led to Ushimaru and the other samurai’s demise.
Things could have gone more or less like this: during Kaido’s invasion of Ringo, Ushimaru asked his trusted retainers to escort his pregnant foreign wife out to East Blue to his clansmen’s place. But Kaido’s forces found them before they escaped and Onimaru (still a fox) stayed back with rest of the retainers to protect his master’s wife. Hence, Zoro’s mom called him Zoro which means fox in Spanish and Roronoa is her family name to keep him safe even though his true name is Shimotsuki Onimaru.
*Theory by KenshiraSonata